Solid-state imaging systems or imaging readers have been installed as electro-optical reading workstations, such as vertical slot scanners, each having a single vertical or upright window, or as bioptical scanners, each having a vertical window and a horizontal window, in supermarkets, warehouse clubs, department stores, and other kinds of retailers and other businesses for many years. This workstation electro-optically reads, by image capture, targets, such as one- and two-dimensional bar code symbols, each bearing elements, e.g., bars and spaces, of different widths and reflectivities, to be decoded, as well as non-symbol targets or forms, such as documents, labels, receipts, signatures, drivers' licenses, employee badges, payment/loyalty cards, and the like, each form bearing alphanumeric characters and graphics, to be imaged. An operator or a customer may slide or swipe a product associated with, or bearing, a target in a moving direction across and past a window of the workstation in a swipe mode. Alternatively, the operator or the customer may momentarily present the target associated with, or borne by, the product to an approximate central region of a window in a presentation mode. The choice typically depends on user preference, the target itself, or on the layout of the workstation.
A known exemplary imaging workstation includes a housing supported on, or incorporated in, a support surface, such as a countertop; the aforementioned upright window supported by the housing and facing the target during reading; and a scan engine or imaging module supported by the housing. The imaging module includes a solid-state imager (or image sensor) with a sensor array of photocells or light sensors (also known as pixels), and an imaging lens assembly for capturing return light scattered and/or reflected from the target being imaged through the window over an imaging field of view, and for projecting the return light onto the image sensor to initiate capture of an image of the target over a range of working distances in which the target can be read. The image sensor may include a one- or two-dimensional charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device and associated circuits for producing and processing electrical signals corresponding to a one- or two-dimensional array of pixel data over the field of view. These electrical signals are digitized, decoded and/or processed by a programmed microprocessor or controller into information related to the target being read, e.g., decoded data indicative of a symbol target, or into a picture of a non-symbol target.
In order to increase the amount of the return light captured by the sensor array, especially in dimly lit environments and/or at far range reading, an illuminating light assembly is typically provided inside the housing for illuminating the target, either continuously or intermittently, through the upright window with illumination light from an illumination light source, e.g., one or more light emitting diodes (LEDs), over an illumination field of view. An object sensing assembly may also be provided inside the housing, for activating or “waking up” the imaging module, e.g., the illuminating light assembly, only if an object or product bearing, or associated with, a target is detected within the imaging field of view. The object sensing assembly has one or more object light sources for emitting object sensing light, typically infrared (IR) light, and at least one object sensor for sensing the return IR light reflected and/or scattered from the object over an object detection field of view.
Although generally satisfactory for their intended purposes, the use of the illuminating light assembly and the object sensing assembly have sometimes proven to be disadvantageous, because a portion of the illumination light and/or of the IR light incident on the upright window of the workstation is reflected therefrom back into the imaging field of view of the image sensor. The reflected portion of the illumination light and/or of the IR light creates undesirable bright or “hot” spots in the imaging field of view, and these hot spots, also known as glare, are specular light, which can overload, saturate, and “blind” the image sensor, thereby degrading reading performance.
In the art of imaging readers, various means have been proposed to eliminate such hot spots caused by reflections of the illumination light and/or of the IR light off the upright window. For example, it is known to configure the upright window as a planar glass window lying in a vertical plane, or in a plane slightly tilted relative to the vertical, at the front of the housing, and to position the illumination LEDs closely adjacent to, and along an upper edge of, the upright window. Since the illumination LEDs in this latter arrangement are positioned well away from the image sensor, the illumination field of view may not be entirely commensurate in scope with the imaging field of view unless multiple LEDS are used to be sure that the illumination field of view will entirely cover the imaging field of view. However, this imposes not only an extra cost burden, but also an electrical power burden. It is desirable to keep electrical power consumption as low as possible, for example, to enable the reader to be powered over a USB cable interface. Furthermore, stray illumination light outside the imaging field of view has to be controlled by the use of light baffles and diffusers. Also, by positioning multiple illumination LEDs close to the upright window, their total illumination is often perceived as bothersome, distracting and annoying to the operators of the readers, and to nearby consumers being served.
As another example, it is also known in the art of imaging readers to configure the upright window with spherical surfaces to prevent the illumination light and/or the IR light incident on the window from reflecting back to the image sensor. This, however, constrains the industrial design of the workstation since, among other things, a spherical window is typically molded from plastic, and not glass.
It is desirable to have the imaging field of view relatively large at a near working distance or a close proximity to the window of the workstation so that the imaging field of view covers the entire target. At farther working distances, it is preferred to have the imaging field of view diverge slowly. An imaging field of view with such characteristics and with a good depth of field is advantageously achieved by making the internal optical path between the imaging module and the window relatively long, and this is typically obtained by inserting at least one fold mirror in this internal optical path to preserve a small, compact volume for the workstation. However, such a fold mirror exacerbates the hot spot problem, because the fold mirror constitutes another reflective surface. The fold mirror and the window form an internal light cavity in which the illumination light and/or the IR light can reflect or “bounce” back and forth, one or more times, back into the imaging field of view of the image sensor, thereby creating additional hot spots and noise, which further degrade reading and waking-up performance.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.